There's more to it then just having the willpower to be committed to a relationship.
I think being committed to a person takes a number of things to make it work. The main ones are communication, happiness, trust, honesty, and work. Even with all of that people can and will still cheat.
To me being able to commit to a relationship is to say, "I want to have this person in my life mean something more then anyone else". You are able to talk to them about most anything (100% everything is too perfect and doesn't exist lol), you can trust them with whatever, they are honest with you, they work with you through the good/bad, the easy/difficult, they work on trying to be happy with you, and most of all they talk with you.
The biggest issues to having such a relationship are fear, a lack of desire and comfort. People that have been hurt in past relationships sometimes come to fear being in future ones no matter how good the partner is to them. Once a person let's fear rule there lives it becomes that much more difficult to make something work.
With having a lack of desire, even though some people see how happy a good relationship can be, they don't want to work for it. It's not easy to make a relationship good or to make it work so some people want it to be easy for them. If you don't have the desire to make a relationship work you won't have a relationship. It takes two people to make something special happen.
When I talk about comfort, I more or less mean comfort in others. People who are able to commit to one person feel comfortable with that person. When they don't feel comfortable with that person they may sometimes seek the comfort of another even if it's just for the physical sense. This isn't a lack of willpower, but rather the relationship has become complacent and one or both parties are needing something to feel safe in. That doesn't mean that the arms of another person is the right way to handle it, but for some who find they cannot communicate with there partners, they find another person easier.
Summary: to make a good relationship work you need to personally make the effort to be committed. Having that mindset to commit will make it that much easier to find another person who will do the same. You may not find them right away and you may get hurt along the way, but all you can do is say that the person you tried to be with wasn't the right one and move on with a lesson learned. Once you find someone you can talk to, trust and communicate with, you can try to work on the relationship with them. No one is 100% perfect and shit will happen, but all you can do is try your best. We are human and alive so we try to make the most out of what life has to offer. In the end you will find what makes you happy regardless if it's a lover, friends, children, animals, etc.
Bonus: I think this manga is a good example of how you try to find happiness for yourself
Oyasumi Punpun. The story ends well, but it shows that love and happiness is different for us all.